Gate-to-ohmic metal contact scheme for III-V devices

Abstract

A direct shorting contact structure comprising a metal layer (18') makes ohmic contact with a source (10) and/or drain (12) region and with a gate electrode (22). The direct contact between source or drain and gate over the active region (16) reduces the area otherwise required for such contact and eliminates a second layer (24) of interconnection metallization otherwise required for such contact. The metal layer preferably comprises a first layer (18'a) of a material selected from the group consisting of gold-germanium, nickel-germanium, gold-germanium-nickel, molybdenum-germanium, and aluminum-germanium in ohmic contact with the source or drain region and with the gate electrode and a second layer (18'b) of a good electrically conductive, thermally stable, electromigration-resistant metal capable of providing good step coverage overlying the first layer. An example of the second layer is tungsten and its nitrides and silicides. Such a composite layer avoids step coverage and reliability problems which exist with other metallization schemes.

What is claimed is: 
     
       1. A method of forming a contact for shorting at least one of a source and drain region to a gate electrode in a III-V semiconductor device having an active region therebetween, said contact comprising an ohmic metal contact, said method comprising contacting at least one of said source and drain regions with a first portion of said contact and contacting said gate electrode with a second portion of said contact, said contact routed directly over said active region between said first portion and said second portion to provide a reduced area contact, said ohmic metal comprising a composite ohmic metal comprising (a) a first layer of a metal which makes ohmic contact to III-V semiconductor materials in contact with at least one of said source or said drain and with said gate electrode and (b) a second layer of an electrically conductive, thermally stable, electromigration-resistant metal capable of providing good step coverage formed over said first layer. 
     
     
       2. A process for forming a contact for shorting at least one of a source and drain region to a gate electrode in a III-V semiconductor device having an active region therebetween, said contact comprising an ohmic contact, said process comprising: (a) forming a patterned gate metal on a surface of a III-V semiconductor wafer;   (b) forming source and drain regions by implantation and activation;   (c) forming a photoresist layer over said surface and said gate metal;   (d) patterning said photoresist layer to expose portions of said source regions, said drain regions, or said source and drain regions;   (e) conditioning said semiconductor surface to receive a first layer of said ohmic contact, said first layer comprising a metal for making ohmic contact to III-V semiconductor materials;   (f) depositing said first layer of said ohmic contact on said patterned photoresist layer;   (g) depositing a second layer of metal on said first layer, said second layer comprising an electrically conductive, thermally stable, electromigration-resistant metal capable of providing good step coverage formed over said first layer; and   (h) removing excess metal on said photoresist layer by liftoff.   
     
     
       3. The process of claim 2 wherein said first layer comprises a material selected-from the group consisting of gold-germanium, nickel-germanium, gold-germanium-nickel, molybdenum-germanium, and aluminum-germanium. 
     
     
       4. The process of claim 3 wherein said first layer is formed to a thickness of about 100 to 1,000Å. 
     
     
       5. The process of claim 2 wherein said second layer comprises a metal selected from the group consisting of platinum, palladium, and chromium, a refractory metal selected from the group consisting of tungsten, molybdenum, and titanium, and the nitrides and silicides thereof. 
     
     
       6. The process of claim 5 wherein said second layer is formed to a thickness of about 500 to 1,700Å. 
     
     
       7. The process of claim 5 wherein said second layer comprises WN x , where x ranges up to about 1.0. 
     
     
       8. The process of claim 5 wherein said second layer comprises WSi x , where x ranges up to about 2. 
     
     
       9. The process of claim 2 wherein a dielectric layer is deposited prior to depositing said photoresist layer and is patterned by etching exposed portions following said patterning of said photoresist layer. 
     
     
       10. The process of claim 2 wherein both said first layer and said second layer are deposited sequentially in situ to avoid exposure of said first layer to the ambient atmosphere. 
     
     
       11. A method of forming a contact for shorting at least one of a source or drain region to a gate contact in a III-V semiconductor device, said contact comprising an ohmic metal contact, said method comprising (1) contacting at least one of said source and drain regions with a first portion of said contact and contacting said gate contact with a second portion of said contact, said contact routed directly between said first portion and said second portion to provide a reduce area contact and comprising a first layer of gold-germanium alloy, and (b) forming a second layer of an electrically conductive, thermally stable, electromigration-resistant metal on said first layer. 
     
     
       12. The method of claim 11 wherein said second layer comprises a metal selected from the group consisting of platinum, palladium, and chromium, a refractory metal selected from the group consisting of tungsten, molybdenum, and titanium, and the nitrides and silicides thereof. 
     
     
       13. A method of forming a contact for shorting at least one of a source or drain region to a gate contact in a III-V semiconductor device having an active region therebetween, said contact comprising an ohmic metal contact, said method comprising (a) contacting at least one of said source and drain regions with a first portion of said contact and contacting said gate electrode with a second portion of said contact, said contact routed directly over said active region between said first portion and said second portion to provide a reduced area contact and comprising a first layer of gold-germanium alloy, and (b) forming a second layer of an electrically conductive, thermally stable, electromigration-resistant metal on said first layer. 
     
     
       14. The method of claim 13 wherein said second layer comprises a metal selected from the group consisting of platinum, palladium, and chromium, a refractory metal selected from the group consisting of tungsten, molybdenum, and titanium, and the nitrides and silicides thereof.